49 lines
1.7 KiB
C
49 lines
1.7 KiB
C
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// Copyright 2020 the V8 project authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
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#define INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
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#include <atomic>
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namespace cppgc {
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namespace internal {
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// A flag which provides a fast check whether a scope may be entered on the
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// current thread, without needing to access thread-local storage or mutex. Can
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// have false positives (i.e., spuriously report that it might be entered), so
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// it is expected that this will be used in tandem with a precise check that the
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// scope is in fact entered on that thread.
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//
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// Example:
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// g_frobnicating_flag.MightBeEntered() &&
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// ThreadLocalFrobnicator().IsFrobnicating()
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//
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// Relaxed atomic operations are sufficient, since:
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// - all accesses remain atomic
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// - each thread must observe its own operations in order
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// - no thread ever exits the flag more times than it enters (if used correctly)
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// And so if a thread observes zero, it must be because it has observed an equal
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// number of exits as entries.
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class AtomicEntryFlag final {
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public:
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void Enter() { entries_.fetch_add(1, std::memory_order_relaxed); }
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void Exit() { entries_.fetch_sub(1, std::memory_order_relaxed); }
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// Returns false only if the current thread is not between a call to Enter
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// and a call to Exit. Returns true if this thread or another thread may
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// currently be in the scope guarded by this flag.
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bool MightBeEntered() const {
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return entries_.load(std::memory_order_relaxed) != 0;
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}
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private:
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std::atomic_int entries_{0};
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};
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} // namespace internal
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} // namespace cppgc
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#endif // INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
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